Crude oil and its fractions are used as feedstock for producing valuable chemicals. Crude oil in oil fields often times forms an emulsion with water.
An emulsion is a mixture of two or more liquids that are normally immiscible where one phase is discontinuously dispersed in the other (“continuous”) phase. There are several types of oil/water emulsions including, water-in-oil (w/o) emulsions [water is the dispersed phase, oil is the continuous phase] and oil-in-water (o/w) emulsions [oil is the dispersed phase, water is the continuous phase], as well as more complex emulsions such as water-in-oil-in-water (w/o/w) emulsions and oil-in-water-in-oil (o/w/o) emulsions. More often than not, the emulsions produced in oil fields are w/o emulsions.
The separation of hydrocarbon gas and liquids and water is generally carried out in vertical or horizontal gravity separators where the two or three phases enter the separator and are separated into separate streams of hydrocarbon gas, hydrocarbon or lighter density liquid and water or heavier density liquid phase. The quality of the liquid streams exiting the separator is affected by the separator efficiency. With less than ideal efficiency or complete separation, some of the heavy liquid phase is carried out with the lighter liquid phase (water-in-oil) and some of the lighter phase is carried out with the heavy phase (oil-in-water). In the process of transportation from the production well to the separator, the fluids mix and are dispersed into one another forming a complex dispersion or emulsion during pipeline transportation from the reservoir that is difficult to separate. The gas phase forms bubbles in the liquids and the liquid phases, hydrocarbon phase (lighter liquid phase) and the water phase (heavier liquid phase) intermingle forming an emulsion or dispersion of droplets of one phase in another.
While most of the gas separates quite easily, the emulsions are typically “tight” or stable and difficult to separate. The gravity separators operate on the principle of providing adequate settling time to the immiscible phases in a relatively quiet horizontal or vertical flow. The emulsion enters the separator and based on the amount of time provided by the capacity of the separation vessel, the phases separate to varying degrees where oil can be found in the water outlet stream and water in the oil outlet stream. Oil droplets rise to the oil-water interface and water droplets settle to the interface. The emulsion layer formed between the phases retards separation. Dispersions of oil and water are complicated by the inhomogeneity of the oil and the impurities in the water. Demulsifiers are added to improve oil-water separation and bottle tests, batch gravity separation, are used qualitatively to assess demulsifier effectiveness to accelerate oil-water separation.
Existing systems and methods for the design and sizing and control of gravity separators use techniques that are based on the retention time of fluids in the separator. These classic design guidelines lead to oversized designs that are costly, or unresponsive to changes in emulsion stability, or undersized and lacking the requisite efficiency. For instance, the retention time design criteria do not take into consideration the inlet conditions, emulsion stability, droplet size distribution, internals, or the water interface level. By way of further example, the design of separators based on the transport of a single droplet or the average retention time of the phase to be separated, typically do not take into account the multidimensional flow field in the separator and is prone to over-sizing the separator volume.
What is needed is a system and method for designing and controlling multiphase separators that allows the determination of the separation efficiency on the basis of the true geometry and multidimensional flow field and for a distribution of droplet sizes with the influence of the emulsion concentration on the rheology of the oil-in-water or water-in-oil dispersion thereby enabling designers, engineers and operators of facilities with separators to determine the phase distribution in the separator, the actual residence or retention time, and to optimize the separator for the prevailing operating conditions.